My U.S. Pat. No. 5,012,690 discloses a powershift transmission for a tractor, the transmission having 18 forward and 9 reverse gear ratios. The transmission has a 3-ratio input clutch section, a 3-ratio center clutch section and a 2-forward, 1-reverse ratio output clutch section. The combination of 3.times.3.times.2 gives 18 forward ratios. However, the arrangement requires multiple clutch swaps to accomplish certain shifts and this in turn requires that internal transmission components must change speed considerably. In the 18.times.9 transmission described in the aforementioned patent, the 10th forward ratio is achieved by engaging the slowest ratio in both the input and center clutch sections and engaging the fast forward ratio in the output clutch section To shift from the 10th to 9th ratio, both the input and center sections must shift to their fastest ratios while the output section shifts to the slow forward ratio. To complete this shift, 3 clutches must engage, 3 other clutches must disengage, and many gears, shafts and clutches in the transmission must speed up thereby absorbing considerable energy. Driving torque through the transmission cannot be maintained throughout the shift thus resulting in a shift that is not as smooth as other gear shifts if the tractor is under load.
The present invention is based on the observation that during most heavy load operations most of the shifting takes place between two or three ratios. The transmission described in the above-referenced patent has the shift sequence 1L 2L 3L 4L 5L 6L 7L 8L 9L 1H 2H 3H 4H 5H 6H 7H 8H 9H where 9L and 1H correspond to the 9th and 10th forward ratios mentioned above. If the high range (H) of ratios is overlapped with the low range (L) to provide the arrangement ##STR1## with the ratio 8L being substantially equal to ratio 1H and ratio 9L being substantially equal to 2H, the number of range shifts may be materially reduced by adopting the shift sequences
______________________________________ Upshift . . . 7L 8L 9L 3H . . . Downshift . . . 3H 2H 1H 7L . . . ______________________________________